Conventionally, for printed wiring boards which are used for mounting of an LED, there have mainly been used rigid metal-clad laminate plates each of which is obtained by subjecting a prepreg which is composed of a sheet-like glass substrate impregnated with a thermosetting resin containing a white pigment to hot-press molding with a metal foil.
Currently, attempts to decrease a thickness and weight of electronic instruments are made for improving portability and appearance of the instruments. In response thereto, chip LEDs are mainly used as light emitting devices for the instruments. However, the chip LEDs are required for reduction in weight and thickness thereof. In addition, substrates having large reflectance and high whiteness are demanded as substrates for white chip LEDs which are used as backlights for a liquid crystal display (LCD) or the like.
However, there have been limits to the reduction in weight and thickness of the conventional rigid metal-clad laminate plates (see, Patent Documents 1 to 3).
Under those circumstances, there is a demand for adhesives and adhesive films having an excellent heat resistance. For example, there is disclosed a method of forming a thermocompression bonding adhesive layer on an insulating substrate by applying a polyimide or polyamic acid dispersion as the adhesive onto the substrate and then removing the solvent, followed by, if required, imidation (see, Patent Documents 4 and 5). In addition, there is disclosed a method of forming a thermocompression bonding adhesive film by applying a polyimide or polyamic acid dispersion onto a glass plate or the like and then removing the solvent, followed by, if required, imidation. To the thus-formed adhesive layer or adhesive film, an adherend such as a metal layer is thermocompression bonded (see, Patent Documents 4 to 6).
For those adhesives and adhesive films having an excellent heat resistance, there are widely used dispersions or films each of which is composed of an aromatic polyimide or an aromatic polyamic acid which is obtained by subjecting an aromatic tetracarboxylic dianhydride and an aromatic diamine to a polymerization condensation reaction. However, although the above-mentioned aromatic polyimide resins have excellent heat resistance and mechanical properties, they absorb a wide spectrum of visible light, resulting in coloration of pale yellow to red-brown. Thus, a resin composition obtained by mixing the aromatic polyimide resin with a white pigment has low reflectance and low whiteness. Therefore, a flexible metal-clad laminate using the resin composition as an adhesive layer has had a problem in that LEDs which are mounted on the laminate exhibit reduced brightness, which is not practical.
It is known that charge transfer between a tetracarboxylic dianhydride moiety and a diamine moiety is suppressed when an aliphatic monomer is used for the monomer constituting a polyimide, resulting in suppression of coloration (see, Non-patent Document 1). Polyimides using the aliphatic monomer are widely utilized for liquid crystal orientation films because of their characteristics, that is, transparency and heat resistance (see, Patent Document 7). However, there has been no report on a flexible metal-clad laminate on which an LED is intended to be mounted, which uses, as an adhesive layer, a resin composition obtained by mixing a polyimide using an aliphatic monomer with a white pigment.
Patent Document 1: Japanese Patent Application Laid-Open No. 2003-60321
Patent Document 2: Japanese Patent Application Laid-Open No. 2003-152295
Patent Document 3: Japanese Patent Application Laid-Open No. Hei 10-202789
Patent Document 4: Japanese Patent No. 2943953
Patent Document 5: Japanese Patent No. 3014526
Patent Document 6: Japanese Patent No. 3213079
Patent Document 7: Japanese Patent Application Laid-Open No. 2001-228481
Non-patent Document 1: “Novel Polyimides: Basics and Applications” edited by Japan Polyimide Conference, NTS Inc., 2002, p. 387-407